1. Field of the Invention
The invention relates to a typewriter ribbon comprising a flexible thin support and a transferable mass placed on the flexible thin support.
2. Description of the Prior Art
Ribbons of the type relating to the present invention are also known as "correctable" typewriter ribbons. These typewriter ribbons are made up of a film base and a thin colored film easily transferable by typing. The advantage of this completely transferable colored film is that the transferred markings, letters and characters, adhere rub-fastedly to the typing paper and can, when needed, be completely lifted by a more or less adhesive correction ribbon. With correctable ribbons it is possible to easily remove wrong or unintentionally typed letters and characters and to add the correct letter or character, or leave the space blank. These typewriter ribbons are mostly used in conjunction with correction ribbons in typing applications. A typewriter equipped with a suitable receiving device for spools and/or cassettes containing the typewriter ribbons and a key and device permitting easy correction, is used.
The first correctable typewriter ribbon was essentially described in DE-OS No. 2 335 838.
The transferable mass of the typewriter ribbon disclosed in DE-OS 2 335 838 contains (1) a film-forming resin, (2) liquid and/or waxy modifying agents incompatible with the resin, and (3) finely dispersed coloring agent. This composition is in the form of a heterogeneous mixture. The main component of the transferable mass is a film-forming resin which has a high degree of flexibility and is not easily friable or flaky. This film-forming resin peels off or crumbles like a wax. It has the advantage that it does not penetrate into the fibers of the typing paper surface and is contact-adhesive. It acts as a carrier, holding the agent or binder for the other components of the colored transferable mass. Typical film-forming resins which perform these functions are, for example, cellulose acetate butyrate, polyester resins, acrylic copolymers and polyamides Particularly a polyamide resin modified by diphenyl acid, and having a softening point of about 98.degree. to 102.degree. C., a viscosity of 2.6 to 3.4 Ns/m.sup.2 at 160.degree. C., an amine number of 5.2 mg KOH/g and an acid number of 2.5 mg KOH/g is used.
So that sharp character definition is obtained after key impact, it is necessary to make the film-forming resin sufficiently easily friable or brittle. This is achieved by lowering the tensile strength of the film formed by the film-forming resin after separation from a solvent. The solvents used are especially in the form of a solvent mixture of toluene and isopropanol having a relatively high toluene content.
An effort should also be made to avoid an excessive softening of the film-forming resin since softening of this resin lowers removability from the paper surface. Modifying agents, which are liquid and/or waxy by nature, are used to lower the tensile strength. Preferred modifying agents for lowering the tensile strength of the resin include mineral oils, which can be chosen within a relatively broad range of viscosities and properties to obtain the necessary degree of modification for each individual resin. Moreover, waxes in the broadest sense, especially synthetic waxes, can also be used. Waxes of this type include synthetic waxes with a base of partially saponified esters of montan wax acids.
Addition of synthetic waxes to the transferable mass has the effect of preventing oil migration to the colored surface or to the carrier if a sizable amount of mineral oil is used as the modifying agent.
Other components of the transferable mass can include softeners or plasticizers of the fatty acid ester type. For example, isopropyl palmitate and butyl stearate or a 2-ethylhexanol ester of a fatty acid mixture (stearic, palmitic and myristic acid) may be used.
Coloring agents which include both solvents or binder soluble and insoluble pigments are, of course, required. Carbon black is a preferred material for coloring.
The transferable mass described above is applied by a solvent coating method to a flexible thin support. The coated flexible thin support is then dried to produce a correctable typewriter ribbon. The flexible thin support is often a polyethylene film.
It has been shown in practice that transferable film typewriter ribbons do not always produce clearly legible, covering typing. Moreover, the type produced is not completely liftable and flawless. The type of impact element (ball element or printwheel), the force of the impact and the size of the font, the surface condition of the typing paper, all decisively affect the quality of the typing product and its correctability.
In particular, papers which have gone through a photocopier and are then to be typed on with a correctable ribbon are problematic in their ability to be typed on and corrected. The cause of this problem with copied paper is due to a change in the surface properties of the paper as a result of the photocopying process. During the process of electrophotographic copying a copy is made visible onto the paper with a coloring agent--also called a toner--from an image that is invisible at first and which is then fixed. Transfer of a fat-like substance occurs with the copying equipment of specific systems. This fat-like substance notably reduces the adhesion of the transferable film to the copied paper.
It is already known that to improve the properties of the heterogeneous mixture, water may be added to the dispersion of the transferable mass. The coating and drying of the transferable mass under specific climatic conditions is also known, but this is unfavorably expensive. Further, the improvements achieved so far are not at all satisfactory.
The typewriter ribbon disclosed in U.S. Pat. No. 3,682 683, which in its essential features corresponds to those described in DE-OS No. 2 335 838, does not solve the problems mentioned above. In particular the problem of typing on copied paper has not yet been solved.
Accordingly, there remains a strong need for a typewriter ribbon which produces intensive, easily readable type onto all kinds of paper, especially photocopied paper. Such a typewriter ribbon should also provide intensive, easily readable type which can be easily removed.